1. Field of the Invention
The present invention relates to a plasma etching process utilized in the fabrication of semiconductor devices, and more particularly, to a method for etching a stacked film comprising a polysilicon film and a metal silicide film using a plasma enhanced etching process.
2. Description of the Related Art
As the integration density of semiconductor devices increases, the line width of conductive layer interconnect patterns forming the semiconductor device gradually decreases. However, a reduction in the line width of conductive layer patterns results in a corresponding increase in line resistance. As a consequence, operating speed is reduced. Hence, a stacked film comprising a polysilicon film and a metal silicide film (hereinafter, referred to as a polycide film) has recently been developed in order to reduce the resistance of the conductive layer, for example a gate electrode or bit line. However, polycide film is subject to various limitations during pattern etching.
In order to form a desired conductive layer pattern (for example, a gate electrode) using a polycide film, first, as shown in FIG. 1, a gate dielectric film 20 is formed on a substrate 10 (which may be an interlayer dielectric layer when the conductive layer pattern is a bit line), and a polycide film comprising a staked polysilicon film 30 and metal silicide film 40, e.g., a tungsten silicide (WSix) is formed thereon. Subsequently, an etch mask 50 having a desired pattern is formed of a material such as silicon nitride.
Next, the polycide film 30+40 is etched through the etch mask 50. To accomplish this, first, the tungsten silicide film 40 is etched using a predetermined first etch gas. Following this, the first etch gas is replaced by a second etch gas for etching the polysilicon film 30. Accordingly, the polysilicon film 30 is etched.
Here, a mixed gas of O2 and Cl2, or a gas obtained by providing a small amount of SF6 in the mixed gas, are typically employed as the etch gas for etching the tungsten silicide film 40. The etch gas has a relatively low etch selectivity of tungsten silicide film 40 with respect to the polysilicon film 30. Accordingly, as shown in FIG. 2, tungsten silicide residue 44 remains on the polysilicon film due to incomplete etching of the tungsten silicide film 42. In this state, the exposed portions of the polysilicon film 32 are partially etched as shown at reference character A. The grooves A formed in the polysilicon film 32 cause non-uniform etching of the polysilicon film 32, leading to substrate 10 damage in some circumstances. Furthermore, as the thickness of the polycide film is reduced in accordance with recent trends toward reducing the total height of semiconductor devices, such grooves A may have a negative impact on the resultant fabricated devices.
An object of the present invention is to provide a method for increasing the etch selectivity of a metal silicide film to a polysilicon film.
Another object of the present invention is to provide a plasma enhanced etching method for a polycide film by which a uniform and suitable etching profile can be obtained.
To achieve the above object, the present invention provides a method of increasing the etch selectivity of a metal silicide film with respect to a polysilicon film. According to the method, after a substrate on which a polycide film has been formed is loaded on a plasma etching chamber, the following steps are performed.
In step (a), a plasma source power is applied to an etch chamber, and a bias power is applied to the substrate. Thus, etch gas ions are accelerated by the applied plasma source power, thereby plasma-etching the metal silicide film.
In step (b), the plasma source power is continuously applied to the etch chamber, and bias power is provided to the substrate at a level at which the etch gas ions are not accelerated. Thus, the etch gas ions are chemically adsorbed to the metal silicide film, and the exposed polysilicon film is oxidized.
In step (c), the plasma source power is continuously applied to the etch chamber, and a bias power is applied to the substrate. Thus, the etch gas ions are accelerated, so that the etch gas ions adsorbed on the metal silicide film during step (b) are etched out, wherein an oxide film formed on the polysilicon film during step (b) is not etched out and serves as a film for preventing etching of the polysilicon film.
The steps (b) and (c) can be repeated until the metal silicide film on the polysilicon film is completely etched.
In preferred embodiment, the cycle of repetition of steps (b) and (c) ranges between 1 Hz to 100 kHz, and the ratio of the duration of the step (c) to the duration of the step (b) ranges from 1:9 to 9:1. The level of bias power applied during step (b) may be substantially equal to zero.
In a method of etching a polycide film using a method of increasing the etch selectivity of a metal silicide film with respect to the polysilicon film, first, the polycide film is formed on a substrate, and an etch mask which is used to etch the polycide film into a predetermined pattern is formed on the polycide film. Next, in a plasma etching chamber, an etch gas which is a mixed gas of Cl2 and O2 is supplied to the polycide film. Next, a plasma source power is applied to the plasma etching chamber, and a pulse-modulated bias power having a first xe2x80x9conxe2x80x9d period and a second xe2x80x9coffxe2x80x9d period is applied to the substrate. Here, the xe2x80x9conxe2x80x9d period is a period during which a level of power at which etch gas ions excited by the plasma source power are accelerated is applied, and the xe2x80x9coffxe2x80x9d period is a period during which a level of power at which the etch gas ions are not accelerated is applied. Thus, during the xe2x80x9conxe2x80x9d period, etch gas ions are accelerated to etch the metal silicide film exposed by the etch mask. During the xe2x80x9coffxe2x80x9d period, the etch gas ions are chemically adsorbed on the metal silicide film, and the exposed polysilicon film is oxidized. Finally, the polysilicon film on the substrate from which the metal silicide film has been completely etched out, and a silicon oxide film formed on the polysilicon film are etched using the etch mask.
In this method, the cycle of the pulse-modulated bias power applied to the substrate preferably ranges from 1 Hz to 100 kHz, and the ratio of the xe2x80x9conxe2x80x9d period to the xe2x80x9con-offxe2x80x9d cycle (xe2x80x9conxe2x80x9d period+xe2x80x9coffxe2x80x9d period) of the bias power preferably ranges from 0.1 to 0.9. The level of bias power applied during the xe2x80x9coffxe2x80x9d period may be substantially equal to zero.
In the present invention as described above, while a bias power is being applied to a substrate, a metal silicide film is etched. On the other hand, when the bias power is not applied, or while the bias power is being applied at a level within a range that does not accelerate etch gas ions, the etch selectivity of a metal suicide film with respect to a polysilicon film can be increased even if an etch gas having a low etch selectivity is used, by forming an oxide film, which is an etching prevention film, on the exposed portions of the polysilicon film. Thus, a uniform and suitable etching profile of a polycide film is obtained.